It is expected that the overall performance of positioning for long term evolution (LTE) will need to be as good as or better than that possible for other access types due to the increasing level of regulatory requirements in some regions and increasing demands imposed by new location service (LCS) applications.
To support these requirements, explicit positioning support should be defined for LTE in a manner compatible with and capable of supporting the emerging 3rd Generation Partnership Project (3GPP) control plane solution and the secure user plane location (SUPL) solution in open mobile alliance (OMA). The overall objective should be to achieve parity with or even surpass the capabilities and performance currently provided for other wireless access types including Global System for Mobile communications (GSM), Wideband Code Division Multiple Access (WCDMA), CDMA2000 1xRTT and CDMA2000 EV-DO.
Moreover, the positioning capabilities and features in association with LTE may support: wireless transmit/receive unit (WTRU)-based and WTRU-assisted observed time difference of arrival (OTDOA) methods, Assisted Global Navigation Satellite System (A-GNSS) methods, enhanced cell identification (ECID), and other methods.
For LTE, the WTRU time difference measurements for the OTDOA method may be based on one or more reference signals (RS) from the serving and/or neighbor cells. The RS may be either the existing Common RS (CRS) and/or a newly designed Positioning RS (PRS). The CRS and PRS may be used individually or in combination by the WTRU to derive the measured metrics. When using PRS, measurement in more than one subframe (called a positioning subframe) may be needed to accumulate enough energy to obtain one measurement sample for one or more specific neighbors.
Various issues may exist with using PRS for positioning measurements.
One issue applicable to both LTE frequency division duplexing (FDD) and time division duplexing (TDD) modes, but particularly a problem in a TDD system where a limited number of downlink (DL) subframes is available, is the availability of N consecutive subframes to derive a measurement.
Another issue may relate to the paging mechanism. In LTE Release 8, a mechanism for paging WTRUs has been defined for WTRUs in idle mode and in connected mode. WTRUs periodically monitor the physical downlink control channel (PDCCH) for downlink (DL) assignments masked with a paging radio network temporary identifier (P-RNTI). On a condition that the assignment is detected, the WTRU demodulates the assigned physical downlink shared channel (PDSCH) resource blocks (RBs) and decodes the paging channel (PCH). This process is called monitoring a paging channel.
In idle mode, a WTRU monitors a paging channel to detect incoming calls, system information changes, and Earthquake and Tsunami Warning System (ETWS) notifications for ETWS capable WTRUs. The specific paging frame (PF) and subframe within that PF (a paging occasion (PO)) that the WTRU monitors are determined based on the WTRU identity (ID) and two parameters specified (directly or indirectly) by the network: paging cycle length (in frames) and the number of paging subframes per paging cycle. A WTRU may receive two paging cycle lengths, a cell-specific one (defaultPagingCycle) and a WTRU-specific one; in idle mode, it uses the smaller of the two. From the network perspective, there may be multiple POs within a PF (i.e., more than one subframe may carry PDCCH masked with a P-RNTI), but the WTRU is only required to monitor one PO per PF, and this PO is determined by the parameters specified above, and provided to the WTRU via broadcast system information and/or dedicated signaling information.
The PRS configuration may be such that an idle mode WTRU may be “blocked” from its POs if all of the following conditions are met: the PRS periodicity is less than or equal to the paging cycle (i.e., the minimum of the cell-specific and WTRU specific paging cycles); any of the frames containing the PRS correspond to the WTRU's PF; and any of the subframes used for the PRS correspond to the WTRU's PO subframe.
In connected mode, a WTRU monitors a paging channel and system information block type 1 (SIB1) contents to detect system information changes, and ETWS notifications for ETWS capable WTRUs. The connected mode WTRU does not need to monitor any specific PO. It simply must try to receive pages at the same rate as a WTRU in idle mode using the cell-specific paging cycle. This rate is determined by a system information block type 2 (SIB2) parameter “modificationPeriodCoeff”. The network will send system information change pages on all POs during a modification period of length modificationPeriodCoeff×defaultPagingCycle.
The PRS configuration may be such that a connected mode WTRU may be “blocked” from at least some of its POs if all of the following conditions are met: the PRS periodicity is less than or equal to the cell-specific paging cycle; any of the frames containing the PRS correspond to any PF; and any of the subframes used for the PRS correspond to any PO subframes.
Another issue may relate to the handling of PRS in subframes allocated for evolved Multicast Broadcast Multimedia Service (eMBMS). The eMBMS feature introduces support for MBMS services into LTE networks. MBMS transmissions are carried over a multicast channel (MCH) that includes multicast shared channel (MSCH), multicast control channel (MCCH), and multicast traffic channel (MTCH). The MCH is mapped to a Physical MBMS Channel (PMCH) that is mapped to MBSFN allocated subframes. In Release 9 eMBMS, the PMCH cannot be multiplexed with the PDSCH into the same subframe. Although eMBMS is a LTE Release 9 feature, the configuration (i.e., the subframe allocation) of the MBSFN allocated subframes was defined for Release 8 to allow Release 8 WTRUs to know which subframes are allocated for MBMS service. In an MBSFN allocated subframe, a Release 8 WTRU decodes the control region (first 1 or 2 symbols) to obtain acknowledgement and negative acknowledgement (ACK/NACK) and uplink (UL) grant information. In MBSFN allocated subframes, the CRS will be present in the control region, but not in the other symbols of the subframe.
In MBMS allocated subframes, in non-control orthogonal frequency division multiplexing (OFDM) symbols, a different RS, the MBSFN RS, rather than the CRS, is used. MBSFN RSs are only defined for the case of extended prefix, i.e., for the case of 6 symbols per timeslot. MBSFN RSs are transmitted in every resource block (RB) in the configured downlink bandwidth in alternating resource elements (REs) in the 3rd symbol of even numbered timeslots and the 1st and 5th symbols of odd-numbered timeslots.
Another issue may relate to the handling of the PRS in subframes containing other RS such as those currently being defined for LTE and LTE-A (LTE advanced). For the purpose of dual-layer beamforming and higher order multiple-input multiple-output (MIMO), Multi-user MIMO (MU-MIMO) and Coordinated Multipoint Transmission (CoMP), additional Demodulation Reference Symbols (DMRS) are being defined. For Release 9, the number of DMRSs will be 12 per RB, and may only be in RBs containing PDSCH, and not in the control region or in symbols containing the CRS.
For LTE-advanced (LTE-A), a total of 24 DMRSs per RB may be used and these DMRS may also not occur in the control region or in symbols containing the CRS. The DMRS on different antenna ports may be multiplexed by frequency division, code division or a combination of frequency and code division.
In addition to DMRS, LTE-A will be adding Channel State Information (CSI)-RS, located throughout the transmission bandwidth of the cell, to allow the WTRU to perform CSI measurements in support of CoMP and MU-MIMO, as well as to support up to 8 DL transmission antenna ports.
Methods and procedures are needed to support PRS in conjunction with WTRU OTDOA based positioning, MBMS, paging mechanisms, available subframes for allocation which may not be consecutive, subframes containing system signals, and subframes containing RSs, in LTE and LTE-A networks.